As high-speed data transmission is required for multimedia communications, multi-carrier modulations such as orthogonal frequency division multiplexing (OFDM) or scalable advanced modulation (SAM) have been developed in order to accommodate these high-speed requirements. OFDM has been found to be an effective data transmission scheme for use with fading and multi-path transmission channels because it reduces inter-symbol interference (ISI) and makes equalization simple. The concept of using parallel data transmission and frequency division multiplexing (FDM) was first published as early as the mid 1960s. These schemes are adopted to avoid the use of high-speed equalization and to combat impulsive noise and multi-path distortion as well as to use the available bandwidth efficiently. In the early 1970s, the application of discrete Fourier transform (DFT) on FDM was discovered to eliminate arrays of sinusoidal generators and coherent demodulation making implementation of OFDM cost-effective. More recently, International Mobile Telecommunications-2000 (IMT-2000) chose MC-CDMA, which is an OFDM application, as a future code division multiple access (CDMA) standard, making the popularity of OFDM grow even more rapidly.
One of the problems associated with OFDM is adjacent channel interference (ACI). The ACI of OFDM is problematic in that the pulse shape of OFDM is rectangular and the spectrum of the pulse is a sum of a sinc function whose sidelobe fades very slowly. Therefore, although the −3 dB bandwidth of an OFDM signal is 10 kHz, its −40 dB bandwidth should be 100 kHz not to interfere with an adjacent channel signal. This problem has been solved generally by using a raised cosine window on the high sidelobe spectrum. However, the raised cosine window cannot reduce the ACI completely with reasonable nonlinear distortion. The ACI problem is not as serious when the number of subcarriers is very large (i.e. >˜1024). However, the ACI problem is very serious when the number of subcarriers is small (i.e. <˜128). The expanded bandwidth by the ACI is relatively large when the number of subcarriers is small and relatively small when the number of subcarriers is large, i.e. the percentage increase of bandwidth depends on the number of subcarriers. That is because the amount of the expanded bandwidth by the ACI is fixed for a given symbol rate. Thus, the seriousness of the ACI problem depends on the number of subcarriers.
The ACI problem is solved dramatically with an isotropic orthogonal transfer algorithm (IOTA) OFDM technique. The IOTA functions, which are the pulse shapes of IOTA OFDM, are not orthogonal to each other when they are spaced by 1/Ts in the frequency domain, as the rectangular pulses, which are the pulse shapes of OFDM, are orthogonal in regular OFDM. However, the IOTA functions are orthogonal to each other when they have real and imaginary data symbol alternations both in time domain and frequency domain as the pulse shapes of Offset QPSK are orthogonal to each other when they have real and imaginary data symbol alternations in time domain. Therefore, IOTA OFDM is a form of OFDM with an offset structure in time and frequency domain. The ACI problem of IOTA OFDM is not so serious because the spectrum of an IOTA function, which is the same with IOTA function in time domain, fades much faster than the spectrum of a rectangular pulse, which is a sinc function. However, its ACI still can be problematic when the number of subcarriers is very small (i.e. <˜32) such as in a wideband radio protocol. To avoid ACI, the signal requires wide edge margins at both sides, which reduces the spectral efficiency of the signal. Therefore, the spectral efficiency of the IOTA OFDM can be worse than SAM because of the problem handling ACI.
SAM is merely an assembly of single-carrier modulations with a root raised cosine (RRC) pulse, which are overlapped slightly in frequency. SAM is spectrally less dense than IOTA OFDM because the RRC is not orthogonal when the subchannels are spaced by 1/Ts in frequency. The subchannel frequency spacing of SAM is (1+α−β)/Ts where α is a roll-off factor of RRC and β is an overlapped amount (α>β). However, a sidelobe of RRC spectrum fades faster than any other pulses. Therefore, SAM has better spectral efficiency than IOTA OFDM when the number of subcarriers is small because its ACI is minimal. But SAM is spectrally less efficient when the number of subcarriers is large because of the wider subcarrier spacing.
Thus, the ACI problem of OFDM that is not solved completely by IOTA OFDM and SAM may be solved by the inventions as discussed herein. The need exists for a new form of OFDM that has the both spectral advantages of IOTA OFDM and SAM while still offering the best spectral efficiency regardless of the number of subcarriers. A new ACI suppression scheme is needed to solve the ACI problem of these multicarrier modulation techniques by reducing the sidelobe without a considerable cost. The new invention should provide an ACI suppression method that uses a modified RRC pulse for a first form of multicarrier modulation and/or a modified IOTA pulse for a second form of multicarrier modulation. The modified RRC also can be applied to reduce the ACI of the single carrier signal as well as that of the multicarrier signal.